1. Field of the Invention
The present invention relates to photon sources comprising at least one emitter capable of being directly or indirectly excited, and thus of emitting a radiation, by successive injection of carriers having opposite polarities (to form on or several localized excitons). Such photon sources include photon sources with quantum boxes such as those comprising nanocrystals in an insulating layer.
2. Discussion of the Related Art
An article by Robert J. Walters et al. entitled “Silicon Nanocrystal Field-Effect Light-Emitting Devices”, published in IEEE Journal of Selected Topics In Quantum Electronics, vol. 12, No 6, November-December 2006, pp. 1697-1656, discloses a light-emitting device with silicon nanocrystals such as illustrated in FIG. 1. This device comprises, on a solid P-type silicon substrate 1, a gate comprising a gate insulator layer 3 incorporating silicon nanocrystals 4. This gate insulator is topped with a polysilicon gate electrode 6 capable of being set to a voltage Vg. Heavily-doped N+-type source and drain regions 7 and 8 are arranged on either side of the gate to form a structure which resembles that of a conventional N-channel MOS field-effect transistor (nMOSFET). In operation, in a first state, the source and the drain are biased so that the component is conductive and electrons are injected via the channel by tunnel effect into nanocrystals 4. In a second state, the source and the drain are set to the same voltage and holes are injected by tunnel effect from the substrate into the nanocrystals. Once they have received an electron and a hole, the nanocrystals emit light.
This light source has a very low intensity, first, basically, because silicon nanocrystals are low-efficiency emitters, and second because the light once emitted needs to cross the gate polysilicon before exiting towards the outside.
An article by Christina Manolatou et al. entitled “Subwavelength Nanopatch Cavities for Semiconductor Plasmon Lasers”, IEEE Journal of Quantum Electronics, vol. 4, No 5, May 2008, pp. 435-447, describes another type of diode-type structure containing a light-emitting medium of type III-V (and not nanocrystals forming quantum boxes). In this article, the authors study the operation of a light-emitting PIN diode inserted in a structure of patch antenna type. This structure is illustrated in FIG. 2. A substrate 11 supports a first metal electrode 12 topped with a PIN diode comprising an N layer 13, an intrinsic layer 14, and a P layer 15, the assembly being topped with another metal electrode 17. The assembly of the two electrodes forms a patch antenna type structure. The article demonstrates that this structure has, when the diode is emitting, the characteristics of a patch antenna and provides a directional radiation. In all the given examples, the PIN layer assembly has a thickness approximately ranging from 140 to 240 nm. Further, the thickness of metal electrode 17 is on the order of 100 nm, which makes it strongly opaque to photons. Thus, the resonant cavity formed between the two metal electrodes has a height (a thickness) on the order of half the wavelength (in the medium present between the two electrodes) of the emitted light. This cavity has a very high finesse, on the order of 200, that is, the source will emit in a very small wavelength range. For example, considering a central wavelength of 800 nm, the cavity may emit within a wavelength range from 796 to 804 nm.
An advantage of such a structure, incorporating a patch antenna type system, is that the outgoing light does not have to cross the upper layers of the photodiode, but exits laterally, to form, under the effect of the antenna, a directional beam.
A disadvantage of the patch antenna type structure provided in C. Manolatou et al.'s article is that, at first sight, it is not suitable to enhance the emission of a structure of the type comprising quantum boxes, given that (1) light source emitter(s) in a quantum box type structure inevitably have different dimensions and generate light within a relatively wide spectral band, for example, from 600 to 1,000 nm in the case of silicon nanocrystals, and that (2) the electromagnetic field concentration is low within the cavity. Thus, only a small number of emitters might be active, and the efficiency of their emission would be low.